The present invention relates to variable displacement compressors of swash plate type provided with a single head piston for use in, for example, air-conditioning systems of vehicles or the like, particularly to variable displacement compressors having special features in the lubrication systems of shaft sealing structures provided between drive shafts (rotary shafts) for driving pistons and their housings.
In a general swash plate compressor of this type, as shown in FIG. 6, its housing is essentially composed of a front housing member 71, a cylinder block 72, and a rear housing member 73 joined and fixed to each other. A drive shaft 74, the front end of which protrudes beyond the front housing member 71, is rotatably supported by the housing through a pair of radial bearings 75 and 76 respectively provided at front and rear portions of the shaft. In the housing, a shaft sealing assembly 78 is provided at a portion nearer to the front end of the drive shaft 74 than the first radial bearing 75. The shaft sealing assembly 78 prevents the leakage of refrigerant gas from a crank chamber 77 to the atmosphere.
In such a compressor, the lubrication for sliding parts such as bearings is effected by lubricating oil, which exists as a mist in the refrigerant gas. Therefore, where the flow of the refrigerant gas is stagnant, the lubrication may become insufficient. Recently, compressors have been proposed for use in refrigerant circuits in which carbon dioxide (CO2) is used in place of chlorofluorocarbon as the refrigerant, and the refrigerant may be cooled in a supercritical region beyond the critical temperature of the refrigerant. When such a refrigerant is used, the refrigerant pressure may become ten or more times higher than that of chlorofluorocarbon refrigerant. Thus, the load on the bearing portions and the shaft sealing assembly increases, and the lubrication must be highly effective.
Japanese Unexamined Patent Publication No. Hei 11-241681 discloses, as shown in FIG. 6, a structure in which a depressurization passage 79 is provided in the drive shaft 74. The inlet 79a of the depressurization passage 79 is open at a position closer to the front end of the drive shaft 74 than the first radial bearing 75 and corresponding to an isolation chamber 80 in which the shaft sealing assembly 78 is accommodated. The outlet 79b of the depressurization passage 79 is open at the rear end of the drive shaft 74. A fan 81 is firmly attached to the end portion of the drive shaft 74 on the outlet 79b side. The fan 81 rotates together with the drive shaft 74, and the refrigerant in the depressurization passage 79 is forced toward the outlet 79b side by the fan 81. The refrigerant discharged on the outlet 79b side then flows through gaps in the radial bearing 76 into the crank chamber 77.
Japanese Unexamined Patent Publication No- Hei 11-107914 discloses a fixed displacement type swash plate compressor that can tolerate a high axial load. In the compressor, as shown in FIG. 7, a suction chamber 82 and a discharge chamber 83 are located on the spline 74a side of a drive shaft 74. A second piston 86 is provided on the opposite side of the spline 74a from a first piston 85 and the first and second pistons sandwich a swash plate 84. In this compressor, the front housing member 71 is provided with an inlet 88 communicating with a swash plate chamber 87 and a connecting passage 89, which connects the swash plate chamber 87 with the suction chamber 82. A shaft seal 90 is located in the suction chamber 82.
In the above-mentioned compressor of Japanese Unexamined Patent Publication No. Hei 11-241681, the operation of the fan 81 creates a refrigerant flow such that some refrigerant from the crank chamber 77 flows through gaps in the first radial bearing 75 or a thrust bearing 91 into the depressurization passage 79 and then returns to the crank chamber 77 through gaps in the second radial bearing 76. Thus, the lubrication of both radial bearings 75 and 76 and the shaft sealing assembly 78 is improved. In this structure, however, since the fan 81 must be provided to make such a refrigerant flow in the depressurization passage 79, the structure is relatively complex.
In the compressor disclosed in Japanese Unexamined Patent Publication No. Hei 11-107914, the suction chamber 82 in which the shaft seal 90 is located is connected with the swash plate chamber 87 by the connecting passage 89. This connecting passage 89 is provided for conducting refrigerant to the suction chamber 82 from the swash plate chamber 87, and it is a typical passage found in fixed displacement type swash plate compressors. In variable displacement type swash plate compressors, however, since the inclination angle of the swash plate (cam plate) is changed to change the displacement by controlling the pressure in the crank chamber, in which the swash plate is located, there is no need to provide such a passage.
The present invention has been achieved in view of the problems described above, and the object of the present invention is to provide variable displacement compressors wherein good lubrication for the shaft sealing assembly for the drive shaft can be effected by a simple structure.
To achieve the foregoing and other objectives and in accordance with the purpose of the present invention, a variable displacement compressor is provided. The compressor includes a housing, a crank chamber, a drive shaft, a cylinder bore, a single head piston, a cam plate, a shaft sealing assembly and a bleed passage. The housing includes a suction chamber and a discharge chamber. The crank chamber is defined in the housing. A first end of the drive shaft extends from a front end of the housing. The shaft is supported by the housing. The suction and discharge chambers are closer to the first end of the drive shaft than the crank chamber. The cylinder bore is located in the housing between the crank chamber and the front end of the housing. The single head piston is located in the cylinder bore. The cam plate is located in the crank chamber and connected with the piston to convert rotation of the drive shaft into reciprocation of the piston. The inclination angle of the cam plate is controlled by controlling the pressure in the crank chamber, to change the discharge displacement. The shaft sealing assembly seals the drive shaft and is located in the suction chamber. The bleed passage connects the crank chamber with the suction chamber. An outlet of the bleed passage is located above the shaft sealing assembly.